The present invention concerns a machine for machining, in particular superfinishing, cylindrical surfaces of circular cylindrical workpieces by moving an abrasive belt tangentially over the cylindrical surface of at least one workpiece in at least one machining station, which abrasive belt, fed from a pay-out spool, passes over a pressure roller before it is rewound onto a take-up spool, the workpiece being supported by a pair of spaced parallel rotary support rollers.
So-called xe2x80x9ccenterlessxe2x80x9d tangential abrasive belt machines comprising one or more fixed machining stations and stationary support rollers advance the workpiece relative to the stationary support rollers either using a pair of support rollers inclined to each other (see for example EP-A-495 691), although this imparts a non-positive advance movement, or positioners mounted on a chain fed with an advance movement, the positioners passing between the pairs of parallel rollers. In this latter case a positive advance movement is certainly imparted to the workpieces but because the rollers are stationary the advance movement of the workpieces on the rollers leads to slipping and therefore rubbing of the workpieces on the rollers. This can degrade the surface condition of the workpieces.
The present invention is directed to a machine of the type defined hereinabove which assures a positive advance movement between the workpieces to be machined and the machining station or stations without slipping and therefore without rubbing of the workpieces on the support rollers.
The machine in accordance with the present invention comprises a support assembly on which the support rollers are mounted so that they can rotate about their axis. The machine further includes a machining assembly comprising one or more machining stations. One of said assemblies is mobile in translation parallel to the axes of the support rollers relative to the other assembly. The machine further includes means for rotating the support rollers about their axis and means for moving one of said assemblies in translation parallel to the axes of the support rollers relative to the other assembly.
In the context of the invention, the mobile assembly can be either the machining assembly carrying the machining unit or units or the support assembly carrying the support rollers, the other assembly being stationary.
The first variant can be particularly advantageous for machining long workpieces and the second variant is more particularly preferable for machining relatively short workpieces.
The machine on which the mobile assembly moves can advantageously comprise at least one pair of support rollers mounted on a table moving in translation.
To exclude any slipping and therefore any rubbing between the workpieces and the support rollers, the support assembly can advantageously carry, in addition to the support rollers, at least one workpiece positioning means designed to act on the workpiece carried by the pair of rollers to position the workpiece axially relative to the support rollers whilst enabling it to rotate.
The positioning means or each positioning means preferably comprises a rotary positioner contributing to the rotation movement of the workpiece during machining. This eliminates or at least reduces slipping and therefore rubbing between the workpiece and the positioner.
The positioning means or each positioning means is preferably disposed at a central position relative to the workpiece, i.e. in a median position relative to the two rollers supporting the workpiece.
The support rollers are preferably rotated by a motor mounted on the table of the mobile support assembly.
The table, which can move in translation on slides, can advantageously be moved in translation by a screw-and-nut type drive system.
The machine preferably comprises a plurality of successive machining stations along the translation direction so that the same workpiece can be subjected to a plurality of successive operations in cascade, for example a plurality of superfinishing operations with increasingly fine grit sizes.